黃果蠅嗅覺受器33c的分子演化

Abstract

果蠅的表皮碳氫化合物具有性費洛蒙的功能，在物種間的識別及生殖隔離方面扮演很重要的角色。果蠅的嗅覺器官為觸角及小顎鬚，先前對多種常見氣味的測試，都未發現小顎鬚上的嗅覺受器33c (Or33c)有任何反應，推測費洛蒙有可能是此受器有反應的未知氣味。另外，在不同物種間，Or33c已被證實受到正向選擇的作用。為了更進一步了解果蠅Or33c的功能以及正向選擇對於Or33c演化的影響，我首先比較了Or33c在不同族群及性別中的表現量，發現Or33c在黃果蠅中的表現具有雌雄雙型性。另外，經由比較Or33c和其縱向複製基因Or33a及Or33b (表現於觸角)的序列多型性，發現無論是在非洲族群或非非洲族群中，Or33c皆受到了正向選汰的作用，且Or33b和Or33c間的基因區段亦受到此正向選汰的作用而發生hitchhiking現象。Or33c具有一個使其能在小顎鬚表現的正向調控模段，此模段的獲得可能與其轉換到不同感受器官上表現及所受之正向選汰有關。了解何種因素驅動了Or33c的正向選汰，將能更進一步釐清Or33c在偵測表皮碳氫化合物上的可能功能。

Pheromonal cuticular hydrocarbons (CHs) have been demonstrated to play important roles in species recognition and sexual isolation in Drosophila. Previous study showed that odorant receptor 33c (Or33c) locating on one of the Drosophila olfactory sense organs, maxillary palpi, may respond to a specific unidentified odorant such as pheromone. In addition, positive selection on Or33c was detected between species. To further investigate the potential function of Or33c in Drosophila melanogaster, the expression level of Or33c between different populations and different sexes were compared, and sexual dimorphism of this gene had been observed. Sequence polymorphisms of Or33c and its tandemly duplicated genes, Or33a and Or33b (expressed on the other olfactory sense organs, antennae), of D. melanogaster populations were also compared to characterize how positive selection shapes the evolution of Or33c in Drosophila. Analyses of the DNA sequences showed that in both African and non-African populations there are significant signatures of positive selection in Or33c. Or33c had acquired cis-regulatory motifs required for the gene expression in the maxillary palp, the acquisition might involve the expression shift to different sensory organs and/or positive selection. Understanding the driving force of positive selection on Or33c will help us investigate the potential role of this gene in the detection of pheromonal CHs.